The most senior figure in nuclear safety has defended the regulation of an atomic power station barely 50 miles from the centre of London that leaked radioactive material for 14 years.

Mike Weightman, chief inspector at the Nuclear Installations Inspectorate, said it was not possible to "inspect or check every feature of a complex plant". But as soon as the leak in the sump of one of the Magnox reactors at Bradwell-on-Sea was discovered the safety body did all it could to ensure that the cause of the problem was identified and dealt with, he added.

The leak became public when a little-publicised case started by the Environment Agency against the then owners of the plant, Magnox Electric Ltd, for 11 breaches of safety regulations came to court last month.

The Environment Agency said it was not willing to talk further about the case because it was sub judice but would explain in detail what it was about when a judgment was finally given.

The lack of information about the leak, which was discovered in 2004, has infuriated environmentalists and risks undermining the government's drive for a new generation of atomic power stations.

Bradwell has been earmarked as a potential site for one of the new plants. It ceased producing electricity in 2002 and is being dismantled by a US company, EnergySolutions.

Weightman, who was confirmed as chief inspector only yesterday after being acting head for five years, told the Guardian the NII operated a "sampling regulatory regime" including inspection that targets those aspects of design and operation that have most significance for safety.

"It is not possible for the regulator to inspect or check every feature of a complex plant," he explained. "In this case the sump was effectively underground and the sump pump was not part of the strict maintenance schedule, and hence would have been most unlikely to have been part of any of our inspection programmes.

"Once we were informed of the leak, which was discovered by the licensee when washing down the sump to address a problem with the sump pump, we instructed the licensee not to use the pump again until the matter was investigated and resolved. We conducted a joint investigation with EA, and agreed afterwards with the licensee measures to determine the extent of the leakage, assess its significance and refurbish the sump to modern standards."

Friends of the Earth said it was wrong that the problems were not made public for five years. "It is extremely unfortunate that ministers have been selling the public a new generation of nuclear power plants since 2005 on the grounds that they are safe when incidents of this kind remain out of the public eye," argued Robin Webster, climate change and energy campaigner at Friends of the Earth.

The power station is said by the Environment Agency to be responsible for allowing liquids to seep into the ground from 1990 to 2004. "It has taken a long time to get to court because it is a complicated case with a lot of detail," said a spokesman for the agency.

Lawyers working for the agency are alleging 11 breaches of the radioactive waste disposal laws by Magnox Electric Ltd, which was originally part of the old Central Electricity Generating Board.

Mark Harris, prosecuting on behalf of the agency, told a jury at Chelmsford crown court that leaks were caused by poor design and continued because of a lack of checks and maintenance.

"The case concerns the disposal of liquid radioactive waste which leaked to the ground from a sump at the site of what is now the former Bradwell nuclear power station," said Harris. "These leaks occurred on a number of occasions between as long ago as 1990 until discovery of these leaks in February 2004.

"In the period when this company was running it ... there was no routine inspection or maintenance of the sump until after the leak was discovered."

EnergySolutions said last night that it had no responsibility for the leak. "We are part of the solution not part of the problem," said a spokesman.